1. Field of Invention
The present invention relates to a continuously variable transmission for a vehicle. More particularly, the present invention relates to a continuously variable transmission for a vehicle which improves power performance and fuel economy as a consequence of realizing two forward speeds and one reverse speed by combining two simple planetary gear sets.
2. Description of Related Art
Generally, continuously variable transmissions used in a vehicle include a shift belt apparatus and a forward/reverse control apparatus.
Torque of an engine is input to the shift belt apparatus through a primary pulley and is output from the shift belt apparatus through a secondary pulley. At this time, continuously variable shifting is achieved by changing radii of the primary pulley and the secondary pulley.
The forward/reverse control apparatus includes single or double pinion planetary gear set and a plurality of friction members controlling the planetary gear set. The forward/reverse control apparatus changes a direction of torque received from the secondary pulley to be forward direction or reverse direction according to the control of the friction members.
The forward/reverse control apparatus may achieve only one forward speed and one reverse speed but may generally achieve two forward speeds and one reverse speed so as to enhance power delivery efficiency and fuel economy.
In such a continuously variable transmission, a conventional forward/reverse control apparatus realizing two forward speeds and one reverse speed is shown in FIG. 1.
The planetary gear set PG included in the forward/reverse control apparatus is a compound planetary gear set of Ravigneaux type, and is formed by combining a single pinion planetary gear set and a double pinion planetary gear set and sharing a ring gear and a planet carrier. Therefore, the planetary gear set PG includes a first sun gear S1, a second sun gear S2, a planet carrier PC, and a ring gear R as rotation elements thereof.
In addition, the second sun gear S2 is directly connected to an input shaft IS so as to be always operated as an input element and the planet carrier PC is directly connected to an output shaft OS so as to be always operated as an output element.
In addition, the first sun gear S1 is connected to a transmission housing H by interposing a first brake B1 therebetween so as to be operated as a selective fixed element, the ring gear R is connected to the transmission housing H by interposing a second brake B2 therebetween so as to be operated as a selective fixed element, and the ring gear R is selectively connected to the planet carrier PC by interposing a clutch C1 therebetween so as to make the planetary gear set PG become direct-coupling state.
As shown in FIG. 2, the second brake B2 is operated at a reverse speed REV, the first brake B1 is operated at a first forward speed D1, and the clutch C1 is operated at a second forward speed D2.
Teeth number of the first sun gear S1 must be smaller than that of the second sun gear S2 in order to reduce difference in gear ratios between the first forward speed and the second forward speed in the compound planetary gear set of Ravigneaux type applied to the forward/reverse control apparatus of the continuously variable transmission. Since the first sun gear S1, the second sun gear S2, the second pinion gear P2, and the ring gear R should be designed to share the first pinion gear P1 of two step gear in common, the difference in the gear rations between the first forward speed and the second forward speed cannot be reduced to be smaller than 2 considering of a limited space for mounting the transmission in the vehicle.
The reason why the difference in the gear ratios between the first forward speed and the second forward speed is reduced to be smaller than 2 is that shift feel may be deteriorated if the difference of the gear ratios is large when shifting. In order to reduce the difference in the gear ratios between the first forward speed and the second forward speed, one or more clutch elements should be added.
In order to reduce the difference in the gear ratio between the first forward speed and the second forward speed by using only three friction members C1, B1, and B2, the first pinion gear P1 should be manufactured such that a portion engaged with the first sun gear S1 and the other portion engaged with the second sun gear S2 are formed at one body and the teeth number of the portion is different from that of the other portion. Since the portion having a smaller exterior diameter interferes with the other portion having a larger exterior diameter when manufacturing the portion, it is very difficult to manufacture the first pinion gear P1 and productivity may be deteriorated.
In addition, the clutch C1 is operated and the planet carrier PC and the ring gear R are connected at the second forward speed. At this time, large torque is applied to the planet carrier PC and it is required for the clutch to have large torque delivery capacity. Therefore, the clutch having large capacity is required and size and weight of the clutch become increase. In addition, degree of design freedom may be deteriorated.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.